A dozen workers will enter reactor unit 1 in shifts of four at a time to install ventilation pipes leading from the reactor hall to a filtration unit in the neighbouring turbine hall. To minimise their exposure to radiation, team members will wear protective suits with a scuba-like air supply, and will stay in the unit for no more than 40 minutes.

The aim is to filter as much radioactive material as possible from the air in the building and replace it with fresh air from outside so that workers can safely spend longer repairing damaged equipment and installing a new system for cooling the fuel in the reactor.

Tokyo Electric Power Company (TEPCO), which owns the complex of six reactor units, said the operation began at 11.30 am local time when two workers wearing breathing apparatus and protective suits entered the building for the first time.

TEPCO says that radiation levels in some parts of unit 1 have fallen to 2 millisieverts per hour. In mid-April, robots sent inside recorded extremely dangerous levels of about 50 millisieverts per hour, way too high to risk human entry. The robots also established that no radioactive water had leaked into the unit, as it had in neighbouring unit 2.

The workers hope to complete their work by Saturday, and have already installed a tent outside the doors to prevent any radioactive air escaping.

Once levels of radioactivity in the air have been driven much lower, workers can begin work on a completely new system for cooling the reactor itself to permanent "cold storage" at 100 °C.

Work is still under way at reactor unit 2 to extract and safely store highly radioactive water, the source of a crisis last month when it began leaking into the sea. On Tuesday TEPCO measured seabed sediments 15 kilometres north of the plant and found 1400 becquerels from radioactive caesium-137 per kilogram of sediment, more than 1000 times higher than normal.

TEPCO plans to bring the whole complex under control within six to nine months.

Meanwhile, it emerged yesterday that many people died in cities struck by the tsunami because instead of following official advice to evacuate on foot to higher ground or tall buildings, they tried to flee in their cars and became stuck in traffic jams.

And to speed up identification of the dead, police in Iwate, Miyagi and Fukushima prefectures revealed yesterday that they will soon begin compiling a database of DNA from surviving relatives of missing people to serve as reference samples. Police will take saliva samples from tens of thousands of people.

Latest police figures reveal that 14,785 are confirmed dead so far, with another 10,271 reported missing.